Duo - the one-to-one video calling app - has now started rolling out. This was announced by the Mountain View, California-based company in a blog post.

"Today, we’re releasing Google Duo — a simple 1-to-1 video calling app available for Android and iOS," the search giant said in the blog post. "Duo takes the complexity out of video calling, so that you can be together in the moment wherever you are."

To refresh, Duo is based on phone numbers, meaning it just requires your phone number to get started, and lets you connect with people in your phone’s contacts list. The app is built to work even if you are on a slower network. It also switches between Wi-Fi and cellular data automatically without dropping your call.

There's also a feature dubbed Knock Knock that allows you to see live video of the caller before you answer the call. Google says it "makes video calling more spontaneous and welcoming," giving you a sense of what the caller is up to and why they want to chat.

The roll-out has just begun, and the app is expected to be available worldwide in the next few days.